The present invention relates to a vinyl chloride resin composition possessing improved transparency, processability and surface smoothness.
While vinyl chloride polymers are widely used because of their excellent physical and chemical properties, there remain various disadvantageous problems in their processability, such as a narrow range of processing temperatures at which forming or molding can be carried out due to only a small difference between the processing temperatures and thermal decomposition temperature of the polymer, and also a slow rate of gelation. It is well known that such drawbacks can be eliminated to some extent by the addition of a plasticizer. However, the addition of a plasticizer not only gives rise to other problems such as evaporation or migration of the plasticizer, but also deteriorates the mechanical properties of formed polymer products.
On the other hand, in order to obtain improved processability so that the formed products of the polymer will have a smooth surface, the rate of gelation will be increased, and deep drawing will be successfully carried out, there have been some attempts to add to a vinyl chloride polymer, as a polymer processing aid, a copolymer having compatibility therewith, such as a copolymer of styrene and acrylonitrile, a copolymer predominantly comprising methyl methacrylate and styrene, or a copolymer predominantly comprising methyl methyacrylate and an alkyl acrylate.
However, all these prior art methods for improving the processability of vinyl chloride polymers have serious drawbacks as follows.
A copolymer of styrene and acrylonitrile impairs the thermal stability of the polymer, and does not significantly improve its processability such as the gelation rate and deep drawing properties.
In comparison with the copolymer of this type, some copolymers predominantly comprising methyl methacrylate and styrene and some copolymers predominantly comprising methyl methacrylate and an alkyl acrylate effectively accelerate the gelation rate and also increase the elongation at a high temperature, whereby satisfactory deep drawing properties can be obtained, and adaptability for fabrication, for example, vacuum forming, is markedly improved. However, ungelled particles (fish eyes) often appear, and the effectiveness in improving the surface smoothness of the formed products such as extruded films is inadequate.
As set forth above, it was difficult with any known polymer processing aids to obtain formed products having highly satisfactory surface smoothness and simultaneously to minimize ungelled particles therein.
According to the knowledge heretofore acquired in the art, it is considered necessary to increase the degree of polymerization of a polymer processing air in order to obtain a formed product having improved surface smoothness on the one hand, and to lower the degree of polymerization of such an aid in order to reduce ungelled particles in a formed product on the other hand. Thus, a method that fully meets these two requirements at the same time has not yet been developed as far as we are aware.